Prior ozone bleaching processes have been performed at either a low pulp consistency or a high consistency. Pulp consistency refers to the water content of the pulp. High consistency pulp is a solid-like, fluff pulp at a consistency of 20% to 45% while low consistency generally refers to a pumpable mixture of about 1% to 5% pulp by weight on an air dried basis.
A low consistency ozone bleaching process uses large volumes of water in contact with the pulp (the pulp constitutes about 1% to 5% by weight of the total) and ozone is diffused or dissolved into the water so that ozone bleaching of the pulp occurs in the water phase.
High consistency ozone bleaching is a gaseous ozone bleaching treatment in which fluff pulp, an apparently solid fibrous material, is contacted with gaseous ozone in a suitable vessel such as the vessel disclosed in U.S. Pat. No. 4,158,597, at an acid pH for sufficient time to bleach the pulp. An ozone bleaching process is disclosed in U.S. Pat. No. 3,451,888 entitled Bleaching Pulp Having High Consistency With Ozone Having Moisture Content Near 100%, and in Canadian Pat. No. 966,604 entitled Kraft Pulp Bleaching and Recovery Process. As disclosed in said Canadian Patent, the high consistency pulp from the gaseous ozone bleaching treatment may be washed if desired and then is treated with alkali in an alkaline extraction treatment or with an alkaline solution of hydrogen peroxide (see page 3, line 22 to page 4, line 11) after ozone bleaching. Washing of the pulp has been suggested prior to alkali treatment but washing displaces the water associated with the pulp during gaseous ozone bleaching while treating with alkali to a pH above 7 destroys the peroxygen values discovered by applicant in such water.
Recycle of effluent from various bleaching treatments to reduce the fresh water requirements of the bleaching operations is well known as disclosed in said Canadian Patent (page 4, lines 17 to 30). However, effluent is not obtained from the gaseous ozone bleaching treatment because the pulp is at a very high consistency (20% to 45%) and appears like a solid fluff. After ozone bleaching, water is added to the ozone bleached fluff pulp in substantial quantities to reduce the consistency to about 1% to 5% in order to achieve a pumpable water/pulp mixture an alkali is immediately added in preparation for the alkaline treatment that immediately follows the gaseous ozone bleaching treatment. This addition of alkali to the pumpable water/pulp mixture destroys the peroxygen values discovered therein by applicant.